Fabrication of polymer neural probes with sub-cellular features for reduced tissue encapsulation

Intracortical microelectrodes currently have great potential as a neural prosthesis in patients with neurodegenerative disease or spinal cord injury. In an effort to improve the consistency of neural probe performance, many modifications to probe design are focused on reducing the tissue encapsulation. Since researchers have shown that small polymer fibers (less than 7-microm diameter) induce a small to non-existent encapsulation layer in the rat subcutis, we have proposed a neural probe design with similarly small diameter structures. This paper discusses the fabrication and design parameters of a microscale neural probe with a sub-cellular lattice structure. We developed a microfabrication process using SU-8 and parylene-C to create the relatively thick probe shank and thin lateral arms. The stiff penetrating shank (70-microm by 42-microm) had an SU-8 core that allowed control over stiffness and simplified the process. Parylene-only structures lateral to the shank could be defined with a 4-microm feature-size to meet our sub-cellular criterion. We fabricated four varying geometries for implantation into the neocortex of seven Sprague-Dawley rats. Our in vivo testing verifies that despite the flexible substrate and small dimensions (4-microm x 5-microm), these devices are mechanically robust and practical as neural probes. These devices provide an important tool for neural engineers to investigate the tissue response around sub-cellular structures and potentially improve device efficacy.